1. Field of the Invention
The present invention relates to a wireless communication device, a wireless communication method, a signal processing device, a signal processing method, and computer program, for performing a receiving operation in a wireless communication system including at least one transmitting antenna and at least one receiving antenna, and more particularly, to a wireless communication device, a wireless communication method, a signal processing device, a signal processing method, and a computer program, for estimating a channel using known information symbols added to the head of a packet.
2. Description of the Related Art
As systems released from the wiring of previous wired communication systems, wireless networks have attracted attention. Examples of a standard for wireless networks include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 and IEEE 802.15. For example, in the IEEE 802.11a/g, an OFDM (Orthogonal Frequency Division Multiplexing) modulation system which is one of multi carrier systems is employed as a standard for a wireless LAN.
In the standard of IEEE 802.11a/g, a modulation system providing a communication speed of 54 Mbps as the maximum limit is supported. However, it is desired to obtain a next-generation wireless LAN standard capable of providing a higher bit rate. A MIMO (Multiple Input Multiple Output) communication attracts attention as a technique for providing an increase in the speed of wireless communications and an OFDM_MIMO communication system is employed in the standard of IEEE 802.11n, which is an extended standard of IEEE 802.11.
The MIMO system is a communication system in which plural antennas are provided to both a transmitter and a receiver so as to embody spatially-multiplexed streams. The transmitter multiplexes plural pieces of transmission data by spatially and temporally encoding the transmission data, distribute the multiplexed transmission data to plural transmitting antennas, and transmits the multiplexed transmission data to a channel. On the other hand, the receiver divides a reception signal into plural pieces of transmission data by spatially and temporally decoding the reception signal received by plural receiving antennas through the channel and thus acquires original data without any crosstalk between the streams. In this MIMO communication system, the frequency band is not widened and the transmission capacity is enhanced depending on the number of antennas, thereby improving communication speed. Since spatial multiplexing is employed, the frequency utilization efficiency is excellent. The MIMO system is a communication system using channel characteristics and is different from a simple transmitting and receiving adaptive array.
In MIMO communication, to spatially divide the spatially-multiplexed reception signal into the original stream signals, it is necessary to acquire a channel matrix H using any method and to calculate a reception weight matrix W used to perform a process of spatially multiplexing and spatially dividing plural streams from the channel matrix H using a predetermined algorithm.
The channel matrix H is acquired by allowing a transmitter and a receiver to transmit and receive a known training sequence, estimating a channel using the difference between the signal actually received and the known sequence, and arranging channel response elements of paths corresponding to combinations of the transmitting and receiving antennas in a matrix form. When the number of transmitting antennas is N and the number of receiving antennas is M, the channel matrix is an M×N (row×column) matrix. Therefore, when receiving N training sequences from the transmitter, the receiver can acquire the channel matrix H using the received training sequences.
An example of a relatively simple algorithm for acquiring the reception weight matrix W from the channel matrix H includes a zero force simply using the inverse matrix H−1 of the channel matrix H as the reception weight matrix on the basis of the logic of completely removing the crosstalk or an MMSE (Minimum Means Square Error) receiving system for calculating the reception weight matrix W from the channel matrix H on the basis of the logic of maximizing the ratio of signal power and squared error (the sum of the crosstalk power and the noise power), that is, SNR. The MMSE is an algorithm intentionally generating the crosstalk and calculating the reception weight matrix W by introducing the concept of the noise power of the receiver. In an environment having great noise, it is known that the MMSE is much better than the zero force. In addition, it is also known that an MLD (Maximum Likelihood Detection) receiving system for estimating the maximum-likelihood transmission sequence by matching with all possible transmission signal sequence patterns is a receiving system having the highest performance. For example, there is disclosed a receiver for decoding multiplexed signals, which are obtained by combining spatial and temporal multiplexing communications using plural antennas for the OFDM modulation, in the MLD system (for example, see U.S. Pat. No. 6,618,454). The MMSE is classified into a waveform equalizing algorithm in a linear region and the MLD is classified into a waveform equalizing algorithm in a nonlinear region.
However, since various RF (Radio Frequency) circuits are incomplete in wireless communications, various error components may be applied to the received symbols. For example, a frequency offset is included in the received symbols due to a reference frequency drift between the oscillators of the transmitter and the receiver.
In a communication system employing the OFDM modulation system, a constant phase error component)(ejΔ) is generated for every OFDM symbol for the frequency offset and the phase error component has an influence on the estimated channel value.
In general, a preamble including repeated known information symbols is added to the head of a packet. When detecting the preamble, the receiver precisely checks the receiving time and normalizes the reception signal power (sets the AGC gain), estimates and corrects the frequency offset, estimates the SNR (Signal to Noise Ratio), estimates the channel, and then demodulates data symbols. However, it is difficult to completely estimate the frequency offset value due to the influence of the noise and the like and the frequency offset component remains at the level of several hundred Hz even after the frequency offset is corrected.
The influence of the residual frequency offset appears as a phenomenon that all sub carriers of each OFDM symbol rotate uniformly depending on the phase error component (ejΔ). When the length of a packet increases, the phase error component (ejΔ) is accumulated and added to every symbol in the residual frequency offset. In a multi-value modulation mode, since a constellation point rotates periodically, it serves as a main factor of the decoding error, which degrades the communication quality.
For example, the residual values of the frequency offset can be estimated using a pilot tone included in a data symbol and the correction can be made by accumulating and adding the residual value in the unit of symbol or over the symbols (widely known).
When a general MIMO receiving algorithm such as an MMSE is used, sufficient decoding performance can be obtained by a countermeasure using the pilot tone. On the contrary, when an MIMO receiving algorithm such as the MLD receiving algorithm in which a great characteristic improvement is expected is employed, the influence of the phase error component due to the residual frequency offset on the estimated channel value appears as the deterioration in the characteristic of a non-negligible level, as has been acknowledged by the inventor.